This invention relates to a process for the scrubbing of particulate material from a gas such as an industrial effluent gas. More specifically, it relates to an improvement in earlier U.S. Pat. No. 3,324,630 entitled "Crossflow Scrubbing Process", and that disclosure is incorporated herein by reference. However, the present invention is not limited in application to the scrubber as described in the aforementioned patent. The process essentially comprises the crossflow application of a scrubbing liquid to a particulate-containing gas stream flowing in an essentially horizontal path through a chamber filled with suitable packing units.
Historically, particles have been removed from gas stream either by expenditure of large quantities of energy, such as in electrostatic precipitators, impinging devices, etc., or by enlarging the cross section of the ducts in which the gases flow to thereby reduce their velocity and achieve more effective settling. Both procedures are characterized by high cost of operation and/or high capital investments.
While some of the prior art processes have relied in whole or in part on the mechanism of "inerital impaction" for achieving the separation of particles from a gas stream, a number of difficulties found inherent in this process create problems of efficiency or cost. The inertial impaction method of particle removal is based on the concept that at a given velocity a suspended particle, by virtue of its higher density compared with the carrier gas, has a greater inertia. Therefore, if an obstruction is present the gas will be diverted around the obstruction whereas the particle, as the result of its greater inertia, will impinge or impact upon the obstruction. This generally results in the separation of the particles from the gas stream by adherence to the obstruction or by the particles dropping out of the gas stream after impact under the force of gravity.
Inertial impaction methods using packed towers for removal of particles from the gas stream have been highly unsuccessful for a number of reasons. For example, the deposition of the particles on the packing causes plugging and inordinately large quantities of flushing liquids are necessary for proper irrigation to remove particles from the packing. Moreover, a high pressure drop is experienced when the gas stream is forced through the packing. Consequently, the excessive size of equipment required, the low efficiency of the process, and the resultant high cost make this method commercially impractical. A maximum of only 60% efficiency was achieved with a Berl Saddle packed column and even that only in a very narrow range of operation [Massey, Chem. Eng. 66(14), 143(1959)].
In the aforementioned U.S. Pat. No. 3,324,630, there is a filamentous a novel process for the removal of particulates from gas streams which comprises a preliminary step for the removal of larger particles on the order of 10 microns or greater followed by passing the gas stream in an essentially horizontal path through one or a plurality of chambers in series packed with a packing material having certain physical characteristics while simultaneously passing a scrubbing liquid in crossflow contact with the gas to achieve particulate nucleation and consequent removal of smaller particles on the order of 0.5-10 microns in size. The preferred packing material is afilamentous packing having little continuous extensive surface and having about 80-85% free volume, and consisting of randomly arranged, interlocked tower packing units, the units being made up of approximately circular, integrally connected filament portions having their axes approximately tangent to a circle at approximately evenly spaced points therearound, the number of such spaced approximately circular portions being from 6-12 and the diameter of such circle being approximately equal to the diameter of one of such approximately circular filament portions plus the diameter of a smaller circle whose circumference is not less than the cross-sectional dimension of the filament portion in the direction of its axis times the number of such filament portions and not greater than the circumference of one of such approximately circular filament portions. This process not only makes it possible to more economically remove smaller particles than any prior art process, but also realizes economies in permitting a more rapid gas flow to be cleaned by a smaller quantity of scrubbing liquid.
A principal feature of the aforementioned process is the discovery that very small particles on the order of about 0.5 microns and smaller can be removed by the process of "nucleation". Nucleation is theorized to consist of two elements, "real" growth by the condensation of moisture on the particles and "effective" growth which is the result of inelastic collisions between wetted particles. This permits very tiny particles to be more effectively removed.
The moisture condensation in the aforementioned process is effected by first increasing the humidity of the dust-laden gas so that it is saturated above the dew point prior to entering the scrubbing unit then using the cooling action of the water or other liquid passing through the scrubbing unit to condense the moisture in the gas on the particles. The increase in humidity is effected by various means such as the introduction of steam, by cooling the gas entering the system, or by direct water spray, etc.
To achieve nucleation and removal of particles on the order of 0.5 microns in accordance with the aforementioned process, it is necessary to employ scrubbing liquid at a temperature of about 40.degree.-80.degree.F. to obtain the requisite cooling action in the scrubbing unit. Because of the high temperature of the gas stream entering the scrubbing unit, generally about 150.degree.F. or higher, the scrubbing liquid is heated by conduction and direct contact with the gas in the process thus requiring cooling, such as in an expensive cooling tower or similar step, before it is suitable for recirculation.